In power conversion, two common failure modes are output overcurrent and output short circuit. Circuit protection arrangements intended to protect against these faults are typically designed to sense the actual output current, for example by measuring the voltage drop across a series sense resistor, power switch, current transformer or the like. These approaches can provide satisfactory protection from overcurrents and short circuits. However, these approaches require additional components which add cost and complexity to the power conversion system. Also, implementations which use components in series with the output, for example a sense resistor, may result in significant additional power dissipation.
In other arrangements, the occurrence of an overcurrent or short circuit may be monitored indirectly, and, in some cases, avoid dissipating excessive additional power. An example of an alternative approach is in the TL5001 Pulse Width Modulation (PWM) Controller made by Texas Instruments. FIG. 1 is a functional block diagram of the TL5001 PWM Controller 10. FIG. 2 is a schematic diagram for a typical power supply circuit 30 using the TL5001 PWM Controller 10.
The diagrams of FIGS. 1 and 2 will be described only to the extent necessary to explain the overcurrent and short circuit protection approach included in the TL5001 PWM Controller.
Referring to FIG. 1, an error amplifier 12 compares a reference voltage 11 with the voltage at a feedback (FB) pin input 13 of the TL5001 PWM Controller 10. Referring to FIG. 2, the input to the FB pin 13 is coupled to the power supply output 15 by a network 17 which sets the gain of the error amplifier 12, and to the compensation (COMP) pin output 16 by a compensation network 9 used in compensating the dc-to-dc converter control loop for stability. Referring again to FIG. 1, in case of a fault, for example if the power supply output 15 is shorted to ground, the error amplifier output 14 output reacts by swinging high to compensate for the low voltage at the power supply output 15. The error amplifier output 14 is coupled to an inverting input 19 of a first short circuit protection (SCP) comparator 20. The first SCP comparator 20 detects the voltage swing and charges an SCP capacitor 21 (FIG. 2) through a current source. The SCP capacitor 21 delays the operation of the fault protection circuitry so that the arrangement does not falsely trigger due to noise, load transients, line transients and the like. This delay will be referred to herein as "fault-delay."
The protection arrangement may include a "soft-start" function in which the operation of the protection arrangement is delayed upon startup to prevent erroneous fault detection due to startup transients. This delay will be referred to herein as "soft-start-delay." In order for the arrangement to function properly, the fault-delay must be longer than the soft-start-delay. If the fault-delay is shorter than the soft-start-delay, then during startup the circuit protection arrangement can trigger after the fault-delay time, even if the soft-start-delay time has not elapsed, and prevent the regulator from starting. This is a limitation of this protection arrangement.
In case of a fault, the SCP capacitor 21 charges to above a predetermined voltage which is monitored by a second SCP comparator 22, and, if a fault is still present, then the second SCP comparator 22 will disable the drive logic and latch the regulator circuit in the off state. The input power must be recycled to restart the regulator circuit.
The fault protection arrangement built into the TL5001 PWM Controller has several limitations. Because the output voltage is input to an error amplifier, the performance of the arrangement is dependent on the characteristics of the error amplifier, e.g. the gain selected. In particular, the trip voltage at which the protection circuit disables the PWM controller may vary depending on the error amplifier characteristic. In case of a temporary fault, it is necessary to recycle input power to restart the PWM controller circuit. Also, the fault-delay time must exceed the soft-start-delay time, or the PWM controller will not start. The protection arrangement used in the TL5001 PWM Controller is designed specifically to be used with the TL5001, and may require modification to be applied to other regulator topologies .
The circuits of FIGS. 1 and 2 are described in the Texas Instruments documents Designing With the TL5001 PWM Controller, SLVA034A, September 1995, and TL5001, TL5001 V Pulse-Width-Modulation Control Circuits, SLVS084S--April 1994--Revised September 1995. The entire contents of both documents are incorporated herein by reference.
There is a need for a fault protection arrangement which can be used with any regulator topology. There is a need for such a fault protection arrangement to have a well defined and consistent trip point in the event of an overcurrent or short circuit fault which trip point does not vary depending upon the characteristics of an error amplifier in the fault protection arrangement. There is further a need for such a fault protection arrangement in which the fault-delay time may be selected to be independent of the soft-start-delay time.